Part of macOS
maildMail in the background
○ Leave it
Fetches and indexes mail for the Mail app.
What it is
Mail’s background service for downloading and searching mail.
If you stop it
macOS starts it again straight away. Leave it.
The verdict
○ Leave it. Part of macOS, or something that starts again by itself. You never need to close these.
In Activity Monitor
maildcom.apple.mail.SpotlightIndexExtension
From Procession’s guide, in the words the app shows.